1. Field of the Invention
The present invention relates to an optical information medium such as read only optical disks and optical recording disks and its fabrication process.
2. Description of the Background
In recent years, optical information media such as read only optical disks and optical recording disks have been required to have ever-higher recording densities and, hence, ever-higher capacities so as to record or store an enormous amount of information such as moving image information. To meet this requirement, media having such high recording densities are now under extensive and intensive research and development.
According to one of approaches proposed so far to this end, recording and reproducing wavelengths are shortened while the numerical aperture (NA) of an objective in a recording and reproducing optical system is increased, thereby reducing the diameter of recording and reproducing laser beams, as typically practiced in the case of a digital versatile disk or DVD. When the recording and reproducing wavelength is decreased from 780 nm down to 650 nm and the NA is increased from 0.45 up to 0.6, the recording capacity of the DVD is 4.7 GB/surface or 6 to 8 times as large as that of a CD.
As the NA increases, however, the tilt margin decreases. The tilt margin is the tolerance of the tilt of an optical recording medium with respect to an optical system, and is determined by the NA. Here let xcex be a recording and reproducing wavelength and t be the thickness of a transparent substrate on which recording and reproducing light is incident. Then, the tilt margin is proportional to:
xcex/(txc2x7NA3)
As the optical recording medium tilts with respect to a laser beam, wavefront aberration (coma) occurs. Here let n denote the refractive index of the substrate and xcex8 stand for the angle of tilt. Then, the wavefront aberration coefficient is given by:
(xc2xd)xc2x7txc2x7{n2xc2x7sin xcex8xc2x7cos xcex8}xc2x7NA3/(n2xe2x88x92sin2xcex8) xe2x88x925/2
From these expressions, it is understood that the thickness t of the substrate should preferably be reduced to increase the tilt margin and prevent the occurrence of coma. In a DVD, indeed, the tilt margin is ensured by making the thickness of the substrate about half (ca. 0.6 mm) the thickness (ca. 1.2 mm) of the substrate of a CD. On the other hand, the thickness variation margin for the substrate is given by:
xcex/NA4 
When there a thickness variation in the substrate, another wavefront aberration (spherical aberration) occurs. Here let xcex94t be the thickness variation of the substrate. Then, the spherical aberration coefficient is given by:
{(n2xe2x88x921)/8n3}xc2x7NA4xc2x7xcex94t
From these expressions, it is understood that to reduce the spherical aberration produced with an increasing NA, it is required to reduce the thickness variation as much as possible. For instance, xcex94t is of the order of xc2x1100 xcexcm in the case of a CD whereas xcex94t is limited to xc2x130 xcexcm in the case of a DVD.
To record moving images of higher quality over an extended period of time, a structure enabling a substrate to become thinner has been put forward in the art. According to this structure, a substrate having an ordinary thickness is used as a supporting substrate for maintaining rigidity. Pits or a recording layer are formed on the surface of the substrate. A light-transmitting layer of about 0.1 mm in thickness is provided as a thin substrate on the recording layer, so that recording and reproducing light can be incident on the recording layer through the light-transmitting layer. With this structure, it is possible to achieve an ever-higher NA and, hence, an ever-higher recording density, because the substrate can be made much thinner than could be possible with conventional structures.
However, it is very difficult to form the light-transmitting layer used for this structure, using resin injection molding. To eliminate such difficulty, it has been proposed to form such a light-transmitting layer by spin-coating of an ultraviolet-curing resin, as typically disclosed in JP-A 9-161333. In JP-A 10-269624, it is proposed to coat a dispersion of spacer particles in a photo-curing resin on a substrate and press the dispersion down on the substrate with a plate material, thereby forming a light-transmitting layer of uniform thickness. In JP-A 10-283683, it is proposed to bond an ultraviolet-curing resin onto a light-transmitting sheet.
When a light-transmitting layer is formed by the processes set forth in the aforesaid publications, however, a medium warping problem arises due to shrinkage upon curing of the resin forming part of the light-transmitting layer. When the photo-curing resin is formed into a film of about 0.1 mm in thickness, it is difficult to achieve uniform curing in the thickness direction. As a result, the light-transmitting layer lacks optical uniformity, and the reliability of the medium is likely to become low due to the presence of uncured monomers. According to the process shown in the aforesaid JP-A 10-283683, the ultraviolet-curing resin layer is thinner than those obtained by other processes because the ultraviolet-curing resin is used as an adhesive layer, and so the medium is less susceptible to warpage. However, the distortion by shrinkage of the resin upon ultraviolet curing leads to another problem that the index of double refraction of the light-transmitting sheet becomes large.
An object of the present invention is to provide an optical information medium comprising a supporting substrate, an information-recording surface provided on a surface of said supporting substrate and a light-transmitting layer provided on said information-recording surface and formed of a resin, wherein said light-transmitting layer is allowed to have a uniform thickness so that optical heterogeneity of the light-transmitting layer, especially an increase in the birefringence of the light-transmitting layer can be reduced, and any warping of the optical information medium is prevented.
Such an object is achievable by the inventions recited below as (1) to (6).
(1) An optical information medium comprising a supporting substrate, an information-recording surface provided on the supporting substrate and a light-transmitting layer provided on the information-recording surface, with recording light and/or reproducing light incident on the information-recording surface through the light-transmitting layer, wherein:
said light-transmitting layer comprises a light-transmitting sheet formed of a resin and an adhesive layer containing pressure-sensitive adhesive for bonding said light-transmitting sheet to an associated side of said supporting substrate.
(2) The optical information medium according to (1), wherein said adhesive layer contains a transparent acrylic resin.
(3) The optical information medium according to (1), wherein said light-transmitting sheet is formed of one resin selected from polycarbonate, polyarylate and cyclic polyolefin.
(4) The optical information medium according to (1), wherein said light-transmitting sheet has been prepared by a casting technique.
(5) The optical information medium according to (1), wherein said light-transmitting sheet has a thickness of 30 to 300 xcexcm.
(6) A process of fabricating an optical information medium as recited in (1), which comprises a step of bonding a light-transmitting sheet larger than said supporting substrate to an associated side of said supporting substrate, and then cutting off a region of said light-transmitting sheet that is unbonded to said supporting substrate by laser processing.